[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Dana Whitlow]

Eric,

Do the observed DAC steps correspond in polarity to the observed frequency
changes, or just the reverse?  That's a key determination to make in placing
blame, for it tells you whether the DAC steps are causative to the frequency
changes, or rather the PLL's reaction to problems in, say, the VCO itself.

Dana


On Tue, Mar 22, 2022 at 2:10 PM Erik Kaashoek <erik at kaashoek.com> wrote:

> Hi Bob,
> By your advice I went for a 6.5 digit DVM and after logging and plotting
> the DAC output its clear there are some stability issues in the DAC
> output. The voltage is wandering around at about the level of frequency
> wandering observed.
> A different supply topology for the DAC and VCXO will have to be
> created.. The DAC resolution is 50 uV (2.048 V / 400000 steps) but the
> random variations are about 4 times p-p larger.
> Once this is done I hope longer term logging of frequency and voltage
> will make sense.
> Or is it better to have a low pass filter between the DAC and the VCXO
> Vtune input so the loop is fast enough to remove the remaining drift?
> If so, what should the time constant of the filter be compared to the
> intersect of the ADEV of the GPS and the VCXO (100 s)?
> Having a big time constant (10s?) will be a pain in the initial tuning
> and will require active components.
> Does owning a 6.5 digit DVM qualify one as a volt-nut? Or should first
> some voltage references be added?
> Erik.
>
> On 5-3-2022 20:50, Bob kb8tq wrote:
> > Hi
> >
> > One suggestion on the frequency plot:
> >
> > TimeLab as you have it set does a “per pixel” process on its charts. You
> have
> > a limited screen resolution and a ton of data. You have to do something.
> There
> > is no way to display it all.
> >
> > It’s a good idea to zoom in to some of the “problem regions” and see
> > what is really going on. This reduces the number of data points that
> > map to one pixel and potentially changes what you see a bit. The closer
> you
> > get to one data point per pixel, the closer you are to “reality” without
> any
> > processing potentially getting in the way.
> >
> > On the DVM, eBay (with some time spent shopping) will sell you a working
> > 6 1/2 digit device for < $100 if you are patient. If you need it now,
> it’s still
> > likely to be below $200. Yes it will be a big clunky box that takes up
> bench
> > space. It will be HPIB and not serial i/o. The display might be a bit
> fun in
> > bright lighting.
> >
> > While you are shopping, I would get set up for phase noise as well. Often
> > it can spot issues that are tough to find with a second to second data
> stream.
> > The setup does not have to be very fancy or terribly expensive.
> >
> > Bob
> >
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